This application claims the priority of Japanese Patent Application No. 2001-95319 filed on Mar. 29, 2001, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a reflection type projection optical system used as a projection optical system in projection display apparatus, projection type TV sets, and the like; and, in particular, to a reflection type projection optical system using a reflecting mirror as an element of the projection optical system.
2. Description of the Prior Art
In projection lenses of projection type display apparatus and projection type TV sets, those of refraction optical systems using optical glass and those combining refraction optical systems and mirrors have widely been known. Most of the projection lenses currently in use are of this type.
When a refraction optical system is used, however, chromatic aberration inevitably occurs due to characteristics of optical glass. In particular, occurrences of axial chromatic aberration and chromatic aberration in magnification may become large problems in a telephoto type having a long focus and a wide-angle type having a short focus, respectively.
When an aspheric optical surface is introduced in order to ameliorate optical performances, many problems may occur due to the fact that optical glass is formed into an aspheric surface. When a large aspheric surface is to be made, in particular, manufacturing problems in terms of cost, weight, and surface accuracy may occur, whereby it will be hard to employ in a product even if it is possible in terms of design. Hence, a projection optical system has been known Japanese Unexamined Patent Publication No. 2000-66105), which is basically constituted by lenses (refraction optical system) whereas large aspheric mirrors are employed in parts necessitating large aspheric surfaces. Though it alleviates problems in terms of cost, weight, and the like when compared with the case where large optical lenses having aspheric surfaces are mounted, the problem of chromatic aberration may be left unsolved.
For eliminating the chromatic aberration generated when using lenses, a projection optical system constituted by three mirrors alone has been known Japanese Unexamined Patent Publication No. 10-111458). The projection optical system using the mirrors alone comprises, successively from the enlargement side, a mirror having a negative power, a mirror having a negative power, and a mirror having a positive power.
The projection optical system using the mirrors alone disclosed in the above-mentioned publication, however, may not be a bright optical system. For making it function as a projection optical system in practice, a very bright illumination optical system may be needed, which increases the size and cost of the illumination optical system.
In view of the foregoing circumstances, it is an object of the present invention to provide a bright reflection type projection optical system while favorably correcting various kinds of aberration such as distortion and chromatic aberration in particular.
The present invention provides a reflection type projection optical system for projecting onto a screen under magnification a luminous flux carrying image information from a light valve driven by an image signal; the reflection type projection optical system comprising successively from the enlargement side:
a first mirror having a positive power;
a second mirror having a negative power; and
a third mirror having a positive power.
Preferably, the reflection type projection optical system satisfies the following conditional expression (1):
xe2x80x83xe2x88x921.0 less than f3/f12 less than xe2x88x920.2xe2x80x83xe2x80x83(1)
where f12 is the composite focal length of the first and second mirrors, and f3 is the focal length of the third mirror.
Preferably, at least one of the three mirrors is constituted by a free-form surface having a form rotationally asymmetrical about an optical axis thereof.
The first mirror may be constituted by a free-form surface having a form rotationally asymmetrical about the optical axis.
Each of the three mirrors may be constituted by a free-form surface having a form rotationally asymmetrical about the optical axis.
All of the three mirrors may have respective optical axes within a single plane.
The three mirrors may have the same optical axis.
The light valve may be one selected from a transmission type liquid crystal device, a reflection type liquid crystal device, and a digital micromirror device.
As mentioned above, the reflection type projection optical system in accordance with the present invention comprises, successively from the enlargement side, a first mirror having a positive power, a second mirror having a negative power, and a third mirror having a positive power.
This yields a favorable balance in terms of aberration correction, thereby ameliorating aberration correction. When negative, negative, and positive mirrors are successively arranged from the enlargement side as in the above-mentioned prior art, the heaviest load is applied to the last mirror (closest to the display device) having a positive power in terms of power distribution, whereby a bright optical system is hard to obtain. When positive, negative, and positive mirrors are successively arranged as in the present invention, by contrast, a positive power can be split into the front and rear sides of the optical system, which is advantageous in terms of aberration correction, whereby a bright optical system can be obtained.